1. Field of the Invention
This invention relates to a radiation detection device, particularly an inorganic scintillation crystal which is optically coupled to a photomultiplier tube (PMT) and which emits detectable light in response to exposure to ionizing radiation. More particularly the invention relates to the use of a radioactive pulser embedded in a second crystal coupled to a second PMT to provide stabilization for the detector device. Such detector devices find use in various industrial and research applications including well logging, high energy physics, aircraft exploration, and the like.
2. Description of Related Art
Devices for detecting and measuring radiation energy have been known for many years. One type of device which is in common use is the scintillation phosphor which employs a substance having the ability to convert ionizing radiation energy into light energy. Such a phosphor is typically coupled to a photomultiplier tube or a photodiode which converts the light energy into an electrical pulse or current. This current is further amplified and is electronically processed to provide useful data concerning the type, strength and other characteristics of the radiation energy.
A number of naturally occurring and synthetically produced materials have been found to possess the property of converting ionizing radiation energy into pulses of light. Among these are organic substances such as anthracene, various synthetic plastics, and naturally occurring as well as doped inorganic crystals such as bismuth germanate, calcium and cadmium tungstate, sodium iodide doped with thallium and calcium fluoride doped with europium.
The most common types of ionizing radiation are gamma or X-rays, alpha particles and beta particles (electrons), having energies ranging from a few thousand electron volts (Kev) to several million volts (Mev).
The different types of scintillators have widely varying characteristics such as density, decay constant, scintillation efficiency, physical strength, hygroscopicity, etc. The pulses of light emitted during scintillation events generally are proportional to the energy deposited in the phosphor. These pulses are detected and counted by a photodetector such as a photomultiplier tube or a photodiode, the latter being used where a strong magnetic field is present, when space is a constraint, or where high energy radiation is being measured and background noise is not a serious problem. Each photomultiplier tube has its own characteristics and it is essential that the right tube be selected for the intended end use application. The current output of the photomultiplier tube is subject to certain variables which necessitate the use of a calibrating standard. Among the variables are temperature, aging or failure of the electronic and other parts, and fluctuations in the power supply.
Prior efforts to stabilize scintillation detectors have involved the use of a flashing light source, an alpha emitting radioactive source such as Am.sup.241 adjacent the scintillator at a point opposite the photodetector, or an alpha source distributed throughout the scintillation crystal. Yet another approach has been to incorporate the radioactive source into a reference crystal to form a pulser, and coupling the pulser to the scintillation crystal opposite the photodetector, as shown in U.S. Pat. No. 3,030,509 dated Apr. 17, 1962. These attempts at standardization all have had drawbacks including thermal instability, low and variable alpha counting efficiency, the need to provide external light sources, etc.